Method and an apparatus for determining the field size and the field form of the radiation cone of ionizing radiation source

ABSTRACT

The object of the invention is a method for determining the size and shape of the radiation field (11) of an X-ray machine automatically by means of a multielectrode ionization chamber (5). By means of the method relating to the invention both the dose and the size and shape of the radiation field can be measured separately so that the ratios of the currents of the electrodes (2, 3) located in different directions in the ionization chamber (5) to the current of the reference electrode (1) provide information on the size and shape of the field, and the sum of the currents is proportional to the dose.

The object of the invention is a method for determining the size andshape of (he field of a radiation beam of an ionizing radiation sourceby means of an ionization chamber.

In recent years, increasing attention has been paid to the X-radiationdoses patients are subjected to in connection with both X-raydiagnostics and the use of treatment equipment. Automatic recording ofradiation doses significantly increases the possibilities for comparingX-ray examinations, thus facilitating the development of X-rayexaminations which reduce the radiation stress on the patient without,however, diminishing the diagnostic value of the examinations.

At present, the radiation dose the patient is subjected to is measuredby means of a so-called area-dosimeter which consists of a flat,bielectrode ionization chamber and an electronic measuring part relatingto it. This equipment gives a reading essentially proportional to theproduct of field size and radiation, but the field size itself cannot beseparately determined. The size and shape of the radiation field are,however, extremely significant, especially in determining thepredisposition of the patient's different organs to radiation.

The aim of the present invention is to determine the size and shape ofthe X-radiation field in a new way. It is characteristic of the methodrelating to the invention

that the field to be measured is divided into at least two separateelectrode areas, so that the first area, or the reference electrodearea, is located mainly in the centre part of the radiation field beingmeasured,

that the edge of the radiation area is set so as to pass through thesecond area, or the electrode area measuring the size and shape of thefield,

and that the shape of the field is determined by comparing the currentof the first electrode area with the current of the second electrodearea.

By means of the method relating to the invention, the size of the usefulbeam of the radiation transmitted from the X-ray tube and limited withscreens can be determined by measuring separately the currents ofdifferent electrodes in a multi-electrode ionization chamber. Bycalculating the proportions of the currents, the size of the field indifferent directions can be determined.

The object of the invention also comprises a device for determining thesize and shape of the field of a radiation beam of an ionizing radiationsource by means of an ionization chamber. At is characteristic of thedevice relating to the invention

that the ionization chamber includes at least two separate electrodes,namely a first electrode, or reference electrode, and a secondelectrode, or the electrode measuring the size and shape of the field,

that the first electrode is located mainly in the centre part of theradiation field being measured, and the second electrode is located sothat the edge of the radiation area can be set to pass through thiselectrode,

that the device includes a measuring part which determines the shape ofthe field by comparing the current of the first electrode with thecurrent of the second electrode.

The ionization chamber relating to the invention differs from aconventional ionization chamber in that the conductive area,corresponding to an electrode gathering positive ions in a conventionalionization chamber, is divided into separate parts. The ionic currentgathered from these areas, that is, from the different electrodes, givesinformation on the ionization of the air or gas space corresponding tothese areas, and thus at the same time on the radiation energy passingthrough the said areas.

The ratio of the currents of the electrodes in the ionization chamberdescribes the size of the field more or less linearly. When aiming atgreater accuracy, however, it is advantageous to calibrate the ratiowith a sufficient number of different values of the radiation field. Thecalibration data obtained is stored in the apparatus' memory, forexample, as a "spline"-type approximation curve, the parameters of whichare calculated automatically in connection with calibration.

The ratios between different electrode currents can also be influencedby changing the shape of the electrodes. Their field dependencies can bemade as linear as possible or appropriately non-linear. Similarly, it isadvantageous to use that part of the measuring chamber, of which thecorresponding electrode area is not used for determining the size of thefield, to measure the ionization current, especially at low radiationvalues. The "protective" electrodes of these areas at the same timeimprove the linearity of the peripheral areas of the electric fieldsformed by polarization voltages.

The invention is described with examples in the following, withreference to the appended drawings in which

FIG. 1 shows diagrammatically an X-raying situation where the radiationdose the patient is subjected to is measured by means of an ionizationchamber.

FIG. 2 shows the electrodes in an ionization chamber according to oneembodiment of the invention, as seen from above.

FIG. 3 shows an ionization chamber relating to the invention as seenfrom the side, as a vertical section along line III--III in FIG. 2.

FIG. 4 corresponds to FIG. 2 and shows the electrodes in an ionizationchamber according to a second embodiment.

FIG. 5 corresponds to FIG. 2 and shows the electrodes in an ionizationchamber according to a third embodiment.

FIG. 1 shows diagrammatically an X-raying situation where the radiationdose the object being X-rayed, that is, the patient, is subjected to ismeasured by means of an ionization chamber 5. The X-raying equipment inFIG. 1 includes an X-ray tube 10, the radiation 11 transmitted fromwhich is limited by screens 14. The radiation passes through the chamber5, a part of the radiation 11 being absorbed by the chamber 5.

The ions formed by the radiation absorbed by chamber 5 pass to differentelectrodes due to the effect of the polarization voltage. The currentsobtained from the electrodes are integrated during the radiation periodin the measuring part 15 to give readings which are proportional to theradiation that penetrated the area corresponding to a part of theelectrode of the chamber 5. The radiation passes further through theobject 12 being X-rayed to the picture receptor 13.

FIG. 2 shows the electrodes in an ionization chamber 5 according to oneembodiment of the invention, as seen from above. In the centre of thechamber 5 is a reference electrode 1 which is presumed to be subjectedto the total amount of radiation. In FIG. 2, the ratio of the current ofelectrode 2, measuring in the horizontal direction, or x-direction, tothe current of the reference electrode 1, is proportional to the size ofthe radiation field in the x-direction. Similarly, in FIG. 2, the ratioof the current of electrode 3, measuring in the vertical direction, ory-direction, to the current of the reference electrode 1, isproportional to the size of the radiation field in the y-direction.

The smaller radiation area 4 and radiation area 6--which is wider in thex-direction--shown in FIG. 2 illustrate typical forms of radiationfields limited with screens 14. When it is presumed that radiation isdistributed evenly in these areas, the current of electrode 3 is foundto be the same with both radiation field forms 4 and 6. The current ofelectrode 2, on the other hand, increases in linear proportion to thewidth of the radiation field in x-direction, when moving from thesmaller radiation field form 4 to the larger form 6.

It is found that the ratio of the current of electrode 2 to the currentof the reference electrode 1, and the ratio of the current of electrode3 to the current of the reference electrode 1, are independent of themagnitude of the radiation. They only change according to the size ofthe radiation field.

In the corners of the ionization chamber 5 of FIG. 2 are also located"protective" electrodes 7, known as such. The current obtained from themcan be added to the current readings of the other electrodes, in whichcase the sum of currents obtained will correspond to the current readinggiven by a conventional chamber, that is, a chamber equipped with asingle, integrated electrode.

FIG. 3 shows an ionization chamber 5 relating to the invention as seenfrom the side, as a vertical section of FIG. 2. Inside the chamber 5, inthe centre of it, is the reference electrode 1, and on both sides of itin the figure the electrodes 2 which measure horizontally, that is, inthe x-direction. Reference number 4 designates the smaller radiationarea shown already in FIG. 2, and reference number 6 refers to the widerradiation area.

FIG. 4 shows a second embodiment of the ionization chamber 5, with onlyone electrode 2 measuring in the x-direction and only one electrode 3measuring in the y-direction. One end of each electrode 2 and 3 is onthe symmetry axis. Due to the structure, the ratio of the current of theelectrode 2 measuring in the x-direction to the current of the referenceelectrode 1 Is directly proportional to the size of the rectangularradiation beam in the x-direction. Similarly, the ratio of the currentof the electrode 3 measuring in the y-direction to the current of thereference electrode 1 is directly proportional to the size of therectangular radiation beam in the y-direction.

FIG. 5 shows a third embodiment of the ionization chamber 5 also withonly one electrode 2 measuring in the x-direction and only one electrode3 measuring in the y-direction. In this example, the electrodes havebeen shaped into wedge form. If necessary, any other shapes can also beused, when one electrode Is to directly indicate, for example, thesurface area.

It is obvious to a person skilled in the art that the invention is notlimited to the embodiments presented in the description and claims, butcan be modified within the scope of the claims. Thus the invention canalso be utilized in other connections than with X-ray sources.

We claim:
 1. A method for determining the size and shape of the field ofa radiation beam of an ionizing radiation source by means of anionization chamber comprising the steps of:dividing the field to bemeasured into at least two electrode areas, a first reference electrodearea located centrally in the radiation field being measured, and atleast one secondary electrode area, setting the edge of the radiationarea so as to pass through said at least one secondary electrode area,and determining the size and shape of the field by comparing the currentof the first reference electrode area with the current of said at leastone secondary electrode area.
 2. A method as claimed in claim 1characterized in that:the ratio of the current of the first electrodearea and the current of the at least one secondary electrode area ismeasured with field shapes of different magnitudes from previously knownmeasurements, and the data obtained is stored in the apparatus memory,the parameters of which are calculated automatically in connection withthe calibration.
 3. A device for determining the size and shape of thefield of a radiation beam (11) of an ionizing radiation source (10) bymeans of an ionization chamber (5), characterized inthat the ionizationchamber (5) includes at least two separate electrodes (1, 2, 3), namelya first electrode, or reference electrode (1), and a second electrode,or the electrode (2, 3) measuring the size and shape of the field, thatthe first electrode (1) is located mainly in the centre part of theradiation field being measured, and the second electrode (2, 3) islocated so that the edge of the radiation area can be set to passthrough this electrode, that the device includes a measuring part (15)which determines the shape of the field by comparing the current of thefirst electrode (1) with the current of the second electrode (2, 3). 4.A device as claimed in claim 3, characterized in that the referenceelectrode (1) is located in the centre of the ionization chamber (5),and the electrodes (2, 3) measuring the shape or the field on eitherside of it.
 5. A device as claimed in claim 3 or 4, characterized inthat in the ionization chamber (5), the reference electrode (1) is inthe centre and four electrodes (2, 3) measuring the size and shape ofthe field are on different sides of it.
 6. A device as claimed in claim3 or 4, characterized in that four separate protective electrodes (7)are located in the corners of the ionization chamber (5).
 7. A device asclaimed in any of claim 3 to 4, characterized in that in addition to thereference electrode (1) and the electrodes (2, 3) measuring the shapeand size of the radiation field, the remaining electrode area acts as aseparate protective electrode (7).
 8. A device as claimed in claim 3,characterized in that the reference electrode (1) is located in thecentre of the ionization chamber (5), and the electrode (2, 3) measuringthe size and shape of the field is located basically between thesymmetry axis and edge of the ionization chamber.
 9. A device as claimedin claim 8, characterized in that in the ionization chamber (5) thereare two electrodes (2, 3) located perpendicular to each other whichmeasure the shape of the field.
 10. A device as claimed in claim 8 or 9,characterized in that the width of the electrode (2, 3) measuring thesize and shape of the field changes from the centre of the ionizationchamber (5) towards its edges.